The present invention relates to a mechanical device which is used in medical applications and which is capable of differentially cutting abnormal deposits from within a patient's vessels.
U.S. Pat. No. 4,445,509 entitled METHOD AND APPARATUS FOR REMOVAL OF ENCLOSED ABNORMAL DEPOSITS which issued to David C. Auth on May 1, 1984 describes a rotary mechanical system for differentially cutting relatively hard intravascular deposits while sparing relatively soft, normal tissue. In the device described in that patent, a hollow channel was used for suction removal of debris generated during the cutting process in order to prevent the debris from acting as the nucleus for thrombogenesis or from occluding smaller vascular channels and thereby inhibiting the normal flow of life sustaining blood.
Suctioning of debris may not recover all of the cutting products if vascular flow is present in the artery being treated, since fluid motion at the cutting tip will immediately carry some debris downstream. U.S. Pat. No. 4,207,874 entitled LASER TUNNELLING DEVICE which issued to D. S. Choy on June 17, 1980 describes an apparatus which removes intravascular deposits by using a laser to vaporize intravascular obstructions. When laser energy is used to vaporize debris, the laser may provide sufficient energy to release each constituent molecule from the host lattice or it may produce gaseous products within the solid matrix, thereby causing a rupture of the matrix and the release of smaller constituent particles of the mass. In the former case, the amount of energy required to uncouple each individual molecule is relatively large due to the binding energy of each molecule and to the large number of molecules per unit volume of obstructing mass. In the latter case, the released particles can be relatively large and may be capable of obstructing smaller vascular branches distal to the site of the treated obstruction.
In U.S. Pat. No. 4,445,509, referred to above, the preferential cutting of hard deposits vis-a-vis soft normal tissue is a desirable feature. Unfortunately, harmful obstructing deposits can, on occasion, be soft. Frequently, such soft occluding deposits are also lacking in physical toughness, i.e., they lack the ability to recover after deformation. Muscular tissue tends to be rather tough and to be able to recover after significant elastic deformation. Thus, an additional physical property which may be considered for differentiating the cutting efficacy of a particular device is its ability to distinguish between soft (compliant), tough tissue, which will not break up as a result of local deformation, and soft, weak tissue, which will break up under local deformation. As taught in U.S. Pat. No. 4,445,509, the differential cutting action derives from the ability of soft tissue to "dive" out of the way before it is caught in front of the cutting edge and cleaved off. The process of "diving" implies deformation which can decimate soft, weak tissue without seriously damaging soft, tough tissue. However, even soft, tough tissue can be cleaved if the rate at which the deformation required to escape cleavage exceeds the speed with which the tissue can move given its own inertia. Thus, increasing the surface speed of the cutting edge can eventually result in the ability to cleave soft, tough tissue. This distinction can be useful when it is desirable to cleave obstructive tissue masses which are soft and weak or soft and tough. Depending upon the local vessel anatomy, some damage to normal vessel endothelium or media may occur, and although less than desirable, that may well be a price worth paying to relieve the underlying obstructive condition. Since damage to endothelium and media occurs routinely in surgical vessel grafts which subsequently re-endothelize, the prognosis for rehealing of intima and media damaged adjacent to removed pathological material is good. Administration of drugs which suppress normal clotting may be required to inhibit thrombosis at the damage site during and after treatment.
When intravascular obstructions have a fibrous structure, there is a tendency to turn up a "scab" of material at the base of the cutting zone. Such scabs grow in size with additional cutting rather than being clipped off. They can present a problem if left within the artery, as they may flop across the arterial vessel and obstruct flow or they may become a nucleating site for thrombogenesis or regrowth of atheroma.